1. Field of the Invention
The invention is in the field of mounts for optical elements, and methods for securing and/or adjusting optical elements.
2. Description of the Related Art
Precision optical devices or elements, such as beam splitters or mirrors, may need substantially strain-free optical mounts to provide diffraction-limited optical performance. One typical type of mount capable of achieving these requirements is referred to as kinematic. The theory of kinematic design assumes that the bodies are perfectly rigid, and that contact only occurs at points. Every rigid body possesses six degrees of freedom. These degrees of freedom are translations along three (3) mutually orthogonal axes, and three (3) rotations around these axes. The theory of kinematic design states that a rigid body has (6-n) degrees of freedom, where (n) is the number of contact points. Any mount or support that constrains a rigid object with more than six (6) contact points, is said to “over-constrain” the object, which likely results in distortion and uncertain position of the object. In kinematic design, three (3) points determine a plane, so that contact of a plane surface to more than three (3) points, distorts the plane surface to the co-planarity of all points contacting the plane surface. This co-planarity requirement for the mating surface must then be equal to the optical tolerance for flatness of the plane surface, which typically is a fraction of the wavelength of light. Such a requirement for mechanical surfaces, designed to mate with optically flat surfaces, imposes significant design and fabrication challenges.
Current mounts use angularly adjustable frames that house rectangular optic devices which are potted with room temperature vulcanization (RTV) silicone, or some other suitable adhesive. These adhesives are liquid when applied and thus conform to the optic's surface, but typically induce some strain into the optic when cured to their final state as a more rigid compound. The coefficient of thermal expansion of these RTV compounds does not equal those of the optic or the metals, such as aluminum, typically used to fabricate optical mounts. To ensure strain-free mounting of potted optics, over temperature changes, the gap used for the potting must be athermalized, i.e., made to function as though it is unchanging, independent of temperature. This means that the gap is sized such that ideally, no stress results from expansion differences between the mount, the RTV or other potting compound, and the optic being mounted. Since materials expand in all three dimensions simultaneously, designing perfectly athermal potting gaps can be challenging. For non-circular optics, such as in rectangular shapes, this more complex geometry results in athermalized gap designs that are typically impractical to fabricate, due to the requirement for continually varying cross-sections. RTV and other optics bonding adhesives generally used, typically possess a low bulk modulus of elasticity to minimize forces on the optic resulting from non-athermalized gaps. The more non-circular the optic, the greater the temperature changes relative to the bonding temperature, the more sensitive the optic to strain, etc., all underscore the importance of a perfectly athermalized potting gap. Strain induced by temperature changes on non-athermalized gaps can both deform the optic, and tilt it relative to surfaces of the frame to which it is potted. As an option, some frames may use flexures for mounting the optic within the frame. The flexure, or any surfaces or materials contacting the optic, which over-constrain it, can create localized stresses at the points of contact-with the optic. This can result in degraded optical characteristics, such as degraded surface figure, with its resulting degradation on optical wavefronts interacting with strained optical surfaces. Minimizing strain in optics can also be challenging, particularly in extreme environmental conditions, such as involving temperature extremes, vibrations, and/or shocks. It would be desirable to have improved optical mounts that would avoid or alleviate these shortcomings.